Re: [PATCH net V3 1/1] net/smc: coordinate wait queues for nonblocking connect

[Eric Dumazet <eric.dumazet@gmail.com>]

On 06/22/2018 07:01 AM, Ursula Braun wrote:
> The recent poll change may lead to stalls for non-blocking connecting
> SMC sockets, since sock_poll_wait is no longer performed on the
> internal CLC socket, but on the outer SMC socket.  kernel_connect() on
> the internal CLC socket returns with -EINPROGRESS, but the wake up
> logic does not work in all cases. If the internal CLC socket is still
> in state TCP_SYN_SENT when polled, sock_poll_wait() from sock_poll()
> does not sleep. It is supposed to sleep till the state of the internal
> CLC socket switches to TCP_ESTABLISHED.
> 
> This patch temporarily propagates the wait queue from the internal
> CLC sock to the SMC sock, till the non-blocking connect() is
> finished.
> 
> In addition locking is reduced due to the removed poll waits.
> 
> Fixes: c0129a061442 ("smc: convert to ->poll_mask")
> Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
> ---
>  net/smc/af_smc.c | 13 +++++++++----
>  net/smc/smc.h    |  1 +
>  2 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
> index da7f02edcd37..7966e7ddb563 100644
> --- a/net/smc/af_smc.c
> +++ b/net/smc/af_smc.c
> @@ -23,6 +23,7 @@
>  #include <linux/workqueue.h>
>  #include <linux/in.h>
>  #include <linux/sched/signal.h>
> +#include <linux/rcupdate.h>
>  
>  #include <net/sock.h>
>  #include <net/tcp.h>
> @@ -605,6 +606,11 @@ static int smc_connect(struct socket *sock, struct sockaddr *addr,
>  
>  	smc_copy_sock_settings_to_clc(smc);
>  	tcp_sk(smc->clcsock->sk)->syn_smc = 1;
> +	if (flags & O_NONBLOCK) {
> +		smc->smcwq = rcu_access_pointer(sk->sk_wq);
> +		rcu_assign_pointer(sock->sk->sk_wq,
> +				   rcu_access_pointer(smc->clcsock->sk->sk_wq));

That is obfuscation.

The following is much easier to read.

	sock->sk->sk_wq = smc->clcsock->sk->sk_wq;

But, this looks very suspect to me.

Nowhere in the stack we divert sk->sk_wq to something else.

What about rcu users of sock->sk->sk_wq ?

		

> +	}
>  	rc = kernel_connect(smc->clcsock, addr, alen, flags);
>  	if (rc)
>  		goto out;
> @@ -1285,12 +1291,9 @@ static __poll_t smc_poll_mask(struct socket *sock, __poll_t events)
>  
>  	smc = smc_sk(sock->sk);
>  	sock_hold(sk);
> -	lock_sock(sk);
>  	if ((sk->sk_state == SMC_INIT) || smc->use_fallback) {
>  		/* delegate to CLC child sock */
> -		release_sock(sk);
>  		mask = smc->clcsock->ops->poll_mask(smc->clcsock, events);
> -		lock_sock(sk);
>  		sk->sk_err = smc->clcsock->sk->sk_err;
>  		if (sk->sk_err) {
>  			mask |= EPOLLERR;
> @@ -1299,7 +1302,10 @@ static __poll_t smc_poll_mask(struct socket *sock, __poll_t events)
>  			if (sk->sk_state == SMC_INIT &&
>  			    mask & EPOLLOUT &&
>  			    smc->clcsock->sk->sk_state != TCP_CLOSE) {
> +				lock_sock(sk);
> +				rcu_assign_pointer(sock->sk->sk_wq, smc->smcwq);
>  				rc = __smc_connect(smc);
> +				release_sock(sk);
>  				if (rc < 0)
>  					mask |= EPOLLERR;
>  				/* success cases including fallback */
> @@ -1334,7 +1340,6 @@ static __poll_t smc_poll_mask(struct socket *sock, __poll_t events)
>  			mask |= EPOLLPRI;
>  
>  	}
> -	release_sock(sk);
>  	sock_put(sk);
>  
>  	return mask;
> diff --git a/net/smc/smc.h b/net/smc/smc.h
> index 51ae1f10d81a..89d6d7ef973f 100644
> --- a/net/smc/smc.h
> +++ b/net/smc/smc.h
> @@ -190,6 +190,7 @@ struct smc_connection {
>  struct smc_sock {				/* smc sock container */
>  	struct sock		sk;
>  	struct socket		*clcsock;	/* internal tcp socket */
> +	struct socket_wq	*smcwq;		/* original smcsock wq */
>  	struct smc_connection	conn;		/* smc connection */
>  	struct smc_sock		*listen_smc;	/* listen parent */
>  	struct work_struct	tcp_listen_work;/* handle tcp socket accepts */
> 


No refcounting when ->smcwq is set ?

This looks quite risky to me.